The light captured by rod and cone cells of the retina triggers phototransduction. This cascade of chemical reactions is similar in both types of cells and is responsible for generating a neuronal signal. The goal of our research is to understand the biochemical mechanisms underlying the restoration of dark conditioned in photo-activated photoreceptor cells. We would like to understand how photolyzed rhodopsin is inactivated by two competing reactions, phosphorylation and reduction of photolyzed chromophore, and how the activated G protein transducin, or G/t, is recycled to its quiescent form. Another important component in the transduction cascade is the restoration of intracellular cGMP concentrations to a pre-illumination level. To understand this process more clearly, we will examine the complex regulation of retinal guanylate cyclase1, the enzyme involved in cGMP production. During the next grant period, we propose to (1) study rhodopsin phosphorylation in rod outer segments under physiological conditions to understand how this reaction is regulated. In addition, we would like to explore the importance of C-terminal modifications, isoprenylation and autophosphorylation, on the enzymatic properties of rhodopsin kinase; (2) identify the Regulator of G-protein Signaling, RGS protein, that is involved in phototransduction; (3) explore the molecular mechanism of guanylate cyclase regulation; and (4) clone and molecularly characterize retinol dehydrogenase from photoreceptor cells.